ublk: fix batch I/O recovery -ENODEV error

ublk: fix batch I/O recovery -ENODEV error

During recovery with batch I/O, UBLK_U_IO_FETCH_IO_CMDS command fails with
-ENODEV because ublk_batch_attach() rejects them when ubq->canceling is set.
The canceling flag remains set until all queues are ready.

Fix this by tracking per-queue readiness and clearing ubq->canceling as
soon as each individual queue becomes ready, rather than waiting for all
queues. This allows subsequent UBLK_U_IO_FETCH_IO_CMDS commands to succeed
during recovery.

Changes:
- Add ubq->nr_io_ready to track I/Os ready per queue
- Add ub->nr_queue_ready to track number of ready queues
- Add ublk_queue_ready() helper to check queue readiness
- Redefine ublk_dev_ready() based on queue count instead of I/O count
- Clear ubq->canceling immediately when queue becomes ready
- Add ublk_queue_reset_io_flags() to reset per-queue flags

Signed-off-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>